Ferroelectric field effect transistor with fluoride buffer and IV-VI ferroelectric

Abstract

There is interposed a buffer film composed of IIa group fluoride and having characteristics of orientation to a surface direction (111), in which mismatching in lattice constant with a crystal element of a semiconductor substrate is large and mismatching in lattice constant with IV-VI group compound ferroelectric substance is small, between the semiconductor substrate having a surface direction (100) and a ferroelectric gate film comprising the IV-VI group compound ferroelectric substance and having characteristics of polarization to the surface direction (111). Since the buffer film is an orientation film in the direction of (111) without influenced by a crystal element of the semiconductor substrate serving as a base material, the ferroelectric gate film can be oriented in the direction of (111) which is the same as the direction of polarization of the ferroelectric substance.

What is claimed is: 
     
       1. A field effect transistor, comprising: a semiconductor substrate of a predetermined first electrically conductive type, including a crystal element having a crystalline orientation (100) on a surface thereof;   impurity diffused layers of a second electrically conductive type opposite to said first electrically conductive type, formed on said surface of said semiconductor substrate, said impurity diffused layers spaced from each other by a predetermined interval and serving as source and drain regions;   a channel region sandwiched between said impurity diffused layers;   a ferroelectric gate film disposed on said channel region and comprising a group IV-VI compound ferroelectric substance having characteristics of crystalline orientation (111) and spontaneous polarization in the direction (111);   a gate electrode disposed on said ferroelectric gate film; and   a buffer film interposed between said surface of said crystal element and said ferroelectric gate film, said buffer film comprising a group IIa fluoride having characteristics of crystalline orientation (111), wherein a degree of mismatch in lattice constant between said group IIa fluoride and said surface of said crystal element is large enough to avoid influence on the characteristics of orientation of said group IIa fluoride by the crystalline orientation (100) on said surface of said crystal element, and a degree of mismatch in lattice constant between said fluoride and said group IV-VI compound ferroelectric substance is small, so that said buffer film and said ferroelectric gate film both have crystalline orientation (111).   
     
     
       2. A field effect transistor according to claim 1, wherein the degree of mismatch in lattice constant between said group IIa fluoride and said group IV-VI compound ferroelectric substance is between almost 0% and 3.5%. 
     
     
       3. A field effect transistor according to claim 1, wherein the degree of mismatch in lattice constant between said group IIa fluoride and said surface of said crystal element of said substrate is at least approximately 6.8%. 
     
     
       4. A field effect transistor according to claim 3, wherein the degree of mismatch in lattice constant between said group IIa fluoride and said group IV-VI compound ferroelectric substance is between almost 0% and 3.5%. 
     
     
       5. A field effect transistor according to claim 1, wherein said buffer film has a uniform fluoride structure with a lattice constant such that: at a side of an interface between said buffer film and said surface of said crystal element of said semiconductor substrate a degree of mismatch in lattice constant between said buffer layer and said surface of said crystal element is large enough to avoid influence on the characteristics of crystalline orientation of said group IIa fluoride by the crystalline orientation (100) on said surface of said crystal element, and   at a side of an interface between said buffer film and said ferroelectric gate film the lattice constant of said fluoride is close to the lattice constant of said group IV-VI compound ferroelectric substance.   
     
     
       6. A field effect transistor according to claim 5, wherein said buffer film is composed of a material selected from the group of materials consisting of SrF 2 , BaF 2  and Sr x  Ba 1-x  F 2  (0<x<1). 
     
     
       7. A field effect transistor according to claim 1, wherein said buffer film has a two-layer fluoride structure, including a first layer having a crystalline orientation (111) and a first lattice constant, said first layer interfacing with said surface of said crystal element of said semiconductor substrate, a degree of mismatch between the first lattice constant and a lattice constant of said crystal element at the interface with the first layer, being large enough to avoid influence on the characteristics of crystalline orientation of said group IIa fluoride by the crystalline orientation (100) on said surface of said crystal element, and   a second layer having a crystalline orientation (111) and a second lattice constant, said second layer interfacing with said ferroelectric film, the second lattice constant being close to the lattice constant of said group IV-VI compound ferroelectric substance.   
     
     
       8. A field effect transistor according to claim 7, wherein said first layer is composed of Sr x  Ba 1-x  F 2  (0<x<1) and said second layer is composed of a material selected from the group of materials consisting of SrF 2  and BaF 2 . 
     
     
       9. A field effect transistor according to claim 1, wherein said buffer film is graded by varying an element ratio in order to vary the lattice constant thereof such that in a vicinity of said surface of said crystal element of said semiconductor substrate, a degree of mismatch between the lattice constants of said crystal element and said group IIa fluoride being large enough to avoid influence on the characteristics of crystalline orientation of said group IIa fluoride by the crystalline orientation (100) on said surface of said crystal element and enforce the (111) orientation of said group IIa fluoride, and   in a vicinity of said ferroelectric film, the lattice constant of the buffer film is close to the lattice constant of said group IV-VI compound ferroelectric substance.   
     
     
       10. A field effect transistor according to claim 9, wherein said buffer film is composed of Sr x  Ba 1-x  F 2 , wherein x has a value which is gradually increased from 0 to 0.5 toward the ferroelectric gate film. 
     
     
       11. A field effect transistor according to claim 1, wherein said semiconductor substrate is a silicon substrate and said crystal element is a silicon crystal. 
     
     
       12. A field effect transistor according to claim 11, wherein a film thickness of said ferroelectric gate film is set from several 100 to 1000 Å, and a film thickness of said buffer film is set from several 100 to 1000 Å. 
     
     
       13. A field effect transistor according to claim 12, wherein said ferroelectric substance is a material selected from the group of materials consisting of GeTe, Pb x  GeTe 1-x  (0<x<1), and GeTe x  Se 1-x  (0<x<1). 
     
     
       14. A field effect transistor according to claim 12, wherein said buffer film has a single-layer structure, composed of a material selected from the group of materials consisting of SrF 2 , BaF 2  and Sr x  Ba 1-x  F 2  (0<x<1). 
     
     
       15. A field effect transistor according to claim 12, wherein said buffer film has a two-layer structure, including a first layer of Sr x  Ba 1-x  F 2  (0<x<1) and a second layer of a material selected from the group of materials consisting of SrF 2  and BaF 2 . 
     
     
       16. A field effect transistor according to claim 12, wherein said buffer film is composed of Sr x  Ba 1-x  F 2  and has a graded structure in which x has a value which is gradually increased from 0 to 0.5 toward the ferroelectric gate film.